A semiconductor light emitting element includes a junction of first-type and second-type semiconductors (e.g., p-type and n-type semiconductors), and is a kind of optoelectronic device in which energy corresponding to bandgap of a semiconductor is emitted in the form of light when voltage is applied to both ends of the first-type and second-type semiconductors. That is, if forward voltage is applied to a p-n junction, electrons of the first-type semiconductor and holes of the second-type semiconductor are injected into the second-type and first-type semiconductors, respectively, and diffused as minor carriers. These minor carriers are recombined with major carriers in a diffusion process, and light corresponding to an energy difference between the recombined electrons and holes is emitted.
Such a semiconductor light emitting element, i.e., a light emitting diode (LED) may emit visible light as well as ultraviolet light. Methods of implementing white light using such semiconductor light emitting element are generally divided into three classes.
First, white light is implemented by combining three LEDs emitting red, green and blue lights, which are the three primary colors of light. In the method, three LEDs should be used for forming one white light source, and a technology for controlling the respective LEDs is required. Second, a blue LED is used as a light source and blue light is partially wavelength-converted by means of a yellow phosphor, thereby implementing white light. In the method, the light emitting efficiency is excellent. However, a color rendering index (CRI) is low and changes depending on current density. Thus, it is difficult to obtain white light close to the sunlight. Last, an ultraviolet light emitting LED is used as a light source and ultraviolet light is wavelength-converted by means of three primary color phosphors, thereby implementing white light. The method can be used under high current and the color tone is excellent. Accordingly, studies thereon have been most actively conducted recently.
Recently, studies have been actively conducted to use such a white LED as a light source for illumination. This is because a semiconductor light emitting element has long life span, small size, lightweight, strong light directivity and low-power driving characteristics, does not require preheating time and complicated driving circuits, and is strong against impacts and vibrations. Therefore, high-quality lighting systems with various forms can be implemented, so that it can be expected to substitute for conventional light sources such as incandescent electric lamps, fluorescent lamps and mercury lamps within 10 years.
However, in order to use such a white LED as a light source substituting for conventional mercury lamps or fluorescent lamps, the whit LED should have excellent thermal stability and emit high-output light even in low power consumption.
As described above, in the case of the semiconductor light emitting element, light is emitted by applying power to the first-type and second-type semiconductor layers joined each other, and allowing electrons and holes to be combined in the junction region. Thus, in order to apply power to the first-type and second-type semiconductor layers, metallic first-type and second-type electrodes are formed on the first-type and second-type semiconductor layers.
However, since the metallic first-type and second-type electrodes do not emit light to the outside but absorbs it, the light output of the semiconductor light emitting element may be lowered by the electrodes.
Thus, it has been tried to enhance the light output of the semiconductor light emitting element by forming a reflective metal layer on one of the first-type and the second-type electrodes, and reflecting the light irradiated to the reflective metal layer, to the outside.
However, in the case where a reflective metal layer is formed on a surface of a semiconductor layer, there are problems in that an interface characteristic of the reflective metal layer with the semiconductor layer is bad to increase contact resistance, and the thermal stability is low to form agglomeration and interface voids in high-temperature heat treatment.